it's copper, tinned copper wire which used to be used in old fashioned fuses

and George has this long bit of fuse wire, which is hung between two electrodes.

You probably can't see so we put some paper on this wire

and you see the line across there and then it comes along here

Well this is a wire which is jittering a bit because ... I don't know why ..

you do just leave me ...

Allright it's hanging there in a shape which doesn't seem to be particularly special.

Now I am going to turn the current on

This is volts going up and this is the current in amperes.

So it goes all the way up and this goes up to 1.8 amps roughly, 1.75

So this is below the point where the fuse wire will melt.

So now, I am going to do the experiment. I am going to turn the current on

and here it goes up to 1.8 amps

and now I am going to turn it off.

on

off

It's reversible: it goes up, it goes down.

on

off

on

off

When the current's on, it goes through the fuse wire

and because of the resistance of the fuse wire

it heats up - that's what a fuse wire is for

it's meant to heat up and melt when you get up to about 200°C

this gets up somewhere near there.

so we are not melting this

We are just heating it up quite a bit

and as we heat it up it expands

and you normally can't see the expansion of a metal when you heat it up

but here you can

So this sagging in the middle is due to the fact that the wire has got

a little bit longer. Less than 1% increase in length will cause it to sag by that amount

So this is a measure of the current going through

That's the first thing, current leads to heat, changes the temperature

leads to expansion, leads to sagging.

Now I've got another magic box

This is a magic box with an N on the top because that's probably something to do with north

and something else on the bottom - south

I am not meant to take this out. George tells me that if I take it out

it will then suck itself into anything and cause great dammage

oh it's a magnet, allright, here it is

another one of these neodymium magnets

it's very, very strong and I am very clumsy with these things

you put near a piece of metal it will disappear into it.

So now I've got a magnet

if I put this horizontally nothing much happens

but if I put it this way

as I move the magnet towards the wire

you can see it's repelled quite strongly

If I turn it rount the other way

so now the north pole is at the bottom

it should pull in the other way, can you see that Brady?

yeah

it comes and hits the magnet

What's going on?

And if I put it horizontally not much happens, well

there is a magnetic field going this way

and when a current goes down the wire this way

that leads to a force on the wire

in a ... not in this direction, not in that direction but in the third direction, perpendicular to it.

And if I turn the magnetic field round I change the direction of the force

so this is the force between a moving charge and a magnetic field

which acts in a funny, funny way. Which you normally can't see

but here it's very graphic cause I got such a strong magnetic field you can actually see it beeing repelled

If I were to be a magician in the 19th century (8, 20th century?)

and I wanted to go on stage and fool people

and I set this up it would look as though I had magic powers, pushing it away

oh, float my beauty

If I now turn the current off

you have a wire without any current

and you have no effect at all, it is proportional to the current

this is sagging a little bit

it would move if it could but it doesn't want to because there's no current.

Pretty cool. Yeah I love this because I mean

thermal expansion used to be one experiment and you could never measure it

and this thing, the force on a moving current, you could never see it, it's so weak

and now I can see both, cause the magnet is so big

So this is, this is, George again to the rescue, this is wonderful, thank you George.

We're talking about the rotation of the earth or other objects and the effects of small forces that can disturb that